1. Field of the Invention
The present invention relates to a high-frequency power amplifier requiring linearity and high efficiency used for multi-level digital modulation communication systems.
2. Description of the Related Art
Recent multi-level digital modulation communication systems etc. often deal with signals which are largely different from each other as concerns average of signal amplitude and the maximum amplitude. Therefore, a power amplifier used for radio communication systems requires linearity and high efficiency. However, when the power amplifier amplifies such signal, the amplifier has generally a lower efficiency, because the operating point of the amplifier is set to be able to amplify a signal to the maximum amplitude without distortion, and accordingly there is little time left to operate near the region having saturation power where a comparatively high efficiency can be maintained.
To solve such problems, various techniques for increasing the efficiency of an amplifier while maintaining linearity have been developed. One of the techniques includes envelope tracking (hereinafter, called “ET”) or envelope elimination and restoration (hereinafter, called “EER”). A basic configuration of an amplifier using these techniques is disclosed in Japanese Patent Laid-Open No. 2005-184273, and National Publication of International Patent Application No. 2006-521081 etc.
Further, a basic configuration of the amplifier is disclosed in “Power Amplifiers and Transmitters for RF and Microwave”, IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 50, NO. 3, MARCH 2002, pp 814 to 826, and “50% PAE WCDMA Base-station Amplifier Implemented with GaN-HFETs”, Compound Semiconductor Integrated Circuit Symposium IEEE, 30 Oct. to 2 Nov. 2005, pp 1 to 4 etc.
In the configurations of those amplifiers, a power supply unit for supplying an amplifying element is configured by, for example, a circuit which operates in switching operation, such as a class S amplifier, using a high-speed switching element to reduce loss. Higher efficiency than that of a normal amplifier such as a class A or B amplifier is realized by externally controlling this high-efficiency power supply unit to fulfill desired ET or EER operation and function, and further high efficiency has been accomplished, also when the power supply unit is outputting with back-off being taken from saturation power.
FIG. 1 is a schematic block diagram illustrating an example of a high-efficiency amplifier to which an EER technique is applied.
An Input signal (modulated signal) is applied to limiter 41 and envelope detector 42. Limiter 41 extracts a phase modulated component signal of the input signal, and envelope detector 42 extracts an envelope signal corresponding to an amplitude modulated component of the input signal. The extracted, phase modulated component signal having constant amplitude is applied to transistor 45 constituting a power amplifier and is amplified.
On the other hand, the extracted envelope signal is applied to a power supply voltage control unit. The power supply voltage control unit includes, as shown in FIG. 1, amplitude-pulse conversion unit 43 including a pulse width modulation (PWM) system etc. for changing a pulse width correspond to an amplitude, switching transistor 44, and a low-pass filter composed of capacitor 46 and choke coil 47. The extracted envelope signal is converted into a voltage corresponding to the envelope signal by the power supply voltage control unit and supplied as an operating power supply voltage for transistor 45 constituting the power amplifier. Amplified signal is outputted from an output terminal of an amplified signal.
As a result, the phase modulated component signal having constant amplitude, extracted in a manner described above, is amplified at high efficiency by transistor 45 constituting the power amplifier, and the power supply voltage for transistor 45 constituting this power amplifier changes according to the voltage supplied by the power supply voltage control unit. Therefore, the signal applied to this power amplifier is amplified so that its amplitude changes correspond to the power supply voltage, and the amplitude modulated component is also reproduced, and therefore a high power amplifier having high efficiency and low distortion is realized.
FIG. 2 schematically illustrates a state in that the power amplifier and the power supply unit in the high-efficiency amplifier shown in FIG. 1 are mounted on a substrate. On the input side of the power supply unit, input terminal 411 for controlling a power supply function is provided. On the input side of the amplifier, input terminal 412 of an amplified signal is provided. The power supply unit is connected to the amplifier through power supply output line 413 for an amplifier.
Generally, the amplifier and the power supply unit are configured as a separate circuit block, and further amplifying element (transistor) 45 that is used for the amplifier and switching element (switching transistor) 44 that is used for the power supply unit are contained individually in a separate package, because of dealing with comparatively large power. Therefore, an area for mounting two packages of the transistor in the substrate is necessary.
Here, as one example, in the configuration described with reference to FIG. 1, the case will be studied in which an amplifier having 90 W saturation power in the band of 2 GHz is configured, using the same transistor elements for transistor 45 and switching transistor 44 that constitute the power amplifier, respectively.
It is assumed that an element containing one field-effect transistor in one package, which is a product “MRF21090” from Freescale Semiconductor, Inc., is selected for a specific transistor having 90 W power. In this case, the outside dimensions of the transistor, except for the electrode portion, are about 34 mm×13.8 mm per unit as shown in FIG. 3. Therefore, the mounting area of a transistor portion requires twice the area defined by these dimensions at a minimum, i.e. a mounting area of about 9.4 cm2.
As mentioned, in the case of the high-efficiency amplifier configured as described above, the mounting area of the transistors for two packages is necessary, because the transistors respectively contained in a separate package are used for the amplifying element and the switching element, which provides a disadvantage in terms of downsizing a device. Also, the configuration provides a disadvantage in terms of lowering the cost of a device.